EGU22-12133
https://doi.org/10.5194/egusphere-egu22-12133
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Exploring the limits of in-situ U-Pb dating of metamorphic garnet

Aratz Beranoaguirre1,2, Leo J. Millonig2, Qiao Shu3, Richard Albert2, Horst R. Marschall2, Jose Ignacio Gil Ibarguchi4, and Axel Gerdes2
Aratz Beranoaguirre et al.
  • 1Karlsruher Institut für Technologie, Karlsruhe, Germany (aratz.beranoaguirre@ehu.eus)
  • 2FIERCE (Frankfurt Isotope & Element Research Center), Goethe Universität, Frankfurt am Main, Germany
  • 3State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China
  • 4University of the Basque Country UPV/EHU, Leioa, Spain

Recent advances in analytical techniques and instrumentation allow for the analysis of increasingly smaller sample volumes and lower concentrations. This development significantly expands the possibilities of in-situ geochronology, e.g., LA-MC-ICPMS. Minerals with low U (and Pb) contents such as garnet become the target of in-situ U-Pb geochronology since ages can potentially be obtained from single (sub-)mm-sized garnet grains in thin sections. In this contribution, we explore the current limits of in-situ U-Pb geochronology: what are the minimum concentrations from which an accurate and precise U-Pb age can be obtained?

For that purpose, we have analysed garnets from three different localities that were unsuccessfully analysed in the past using a single-collector sector-field Element XR instrument at FIERCE. These garnets have been re-analysed at FIERCE using a Neptune Plus MC-ICPMS coupled to a RESOLution-LR ArF Excimer laser. The analyses were performed in static mode measuring the masses 206Pb and 207Pb with Secondary Electron Multiplier (SEM) and 202Hg, 204Pb and 238U with the Multiple Ion Counters (MIC). With a spot diameter of 193 μm (round) and a fluence of 2 J/cm2 at 15 Hz, ca. 18 µm pit depth was ablated in 18s analysis time, resulting in a total of 2 µg of ablated material. This is more than 2,000 times less material compared to conventional isotope dilution analyses and 3,000 times less U than for a typical LA-ICPMS zircon analysis (20 µm spot). Although the analysed garnets typically have U contents below 10 ng/g, about 15–30 spots are commonly sufficient to define a regression line in the Tera-Wasserburg diagram, yielding a precision of typically <3 % for the lower intercept age. Challenges and details of the method will be discussed using samples of metamorphic garnet from Kaapvaal craton granulites and Eastern and Western Variscan eclogites.

How to cite: Beranoaguirre, A., Millonig, L. J., Shu, Q., Albert, R., Marschall, H. R., Gil Ibarguchi, J. I., and Gerdes, A.: Exploring the limits of in-situ U-Pb dating of metamorphic garnet, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12133, https://doi.org/10.5194/egusphere-egu22-12133, 2022.